Final answer:
The appropriate preterite verb to fill in the blank is visitamos.
Step-by-step explanation:
The appropriate preterite verb to fill in the blank is visitamos. The sentence would read: Olga y yo visitamos a Gerardo ayer. The verb 'visitar' means 'to visit' in English, and in the preterite tense, 'visitamos' means 'we visited'.
Learn more about Preterite tense in Spanish